Water quality and pollution by oxygen consuming substances

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Last modified 18 Feb 2008, 12:34 PM

Organic matter, measured as Biochemical Oxygen Demand (BOD) and ammonium, are key indicators of the oxygen content of water bodies.

Concentrations of these determinands normally increase as a result of organic pollution caused by discharges from waste water treatment plants, industrial effluents and agricultural run-off. Severe organic pollution may lead to rapid de-oxygenation of river water, a high concentration of ammonia and the disappearance of fish and aquatic invertebrates.

The most important sources of organic waste load are: household waste water; industries such as paper industries or food processing industries; and occasionally silage effluents and slurry from agriculture. Increased industrial and agricultural production, coupled with a greater percentage of the population being connected to sewerage systems, initially resulted in increases in the discharge of organic waste into surface water in most European countries after the 1940s. Over the past 15 to 30 years, however, the biological treatment of waste water has increased, and organic discharges have consequently decreased throughout Europe.

For example, up until the early 1970s the Rhine was polluted with such excessive amounts of organic matter that in the central and lower reaches the river was virtually dead. The concentration of organic matter measured as BOD and ammonia reached high levels. The annual average oxygen concentration was around 5 mg O2/l and the number of invertebrate species reached a critically low level in 1971. Over the past 30 years biological treatment of waste water has increased, and the organic loading has consequently decreased. The result is that the BOD and ammonium concentrations are low, and the oxygen conditions have improved. As a result, the number of invertebrate species has recovered.

Trend in A) ammonium concentration and B) aquatic community composition and oxygen concentration of the Rhine at the German-Dutch border

Concentrations of BOD and total ammonium have decreased in European rivers in the period 1992 to 2004, corresponding to the general improvement in wastewater treatment. The decrease is due mainly to improved sewage treatment resulting from the implementation of the Urban Wastewater Treatment Directive. The economic recession of the 1990s in central and eastern European countries also contributed to this fall, as there was a decline in heavily polluting manufacturing industries.

BOD and ammonium concentrations are highest in some central European countries and lowest in the north. The EU-10 Member States have witnessed the greatest declines since 1992.

BOD_NH4: Trend in BOD5 and total ammonium concentrations in rivers between 1992 and 2004.